There is a well-established epidemiological association between the incidence of affective episodes and suicides and the seasons. Our group has previously described patients who regularly become depressed each winter, and we have called this syndrome Seasonal Affective Disorder (SAD). One approach to understand how changes in mood in this population are altered by seasonal changes in the environment is to study physiological and biochemical parameters at different times of the year in people whose mood is vulnerable to seasonal changes, and in healthy controls. This may yield insights into the mechanism of seasonal influences on the incidence of affective episodes in the general population. In eight patients we showed that during the winter, sleep length was significantly increased (a replication of an earlier finding), REM density was significantly increased and delta sleep tended to be decreased. Several summer-winter studies on small numbers of patients showed no seasonal effect. These included basal metabolic rate (N=10), glucose tolerance tests (N=7), and urinary 6-OH-melatonin (N=6). In only 2 out of 13 dexamethasone suppression tests (DSTs) performed on SAD patients during the winter was there a failure to suppress cortisol secretion normally. This concurs with our previous finding that in the winter depressions of SAD patients DSTs are generally normal. In a preliminary study, hourly overnight plasma samples were drawn in four SAD patients and three normal controls in winter before and after a week of morning and evening light treatment. Patients showed higher baseline levels of melatonin and appeared less sensitive to the melatonin-suppressing effects of light than the volunteers. We are planning to follow up this interesting preliminary finding with systematic circadian studies of melatonin and cortisol in summer and winter in a larger number of patients and normals.